Certain output driver circuits in telecommunications integrated circuits drive highly loaded transmission lines or telephone cables. Such an output driver amplifies a relatively weak input signal to provide an output signal. The output driver must provide the output signal on the transmission line with a minimum amount of distortion while consuming the least amount of power. In order to minimize power consumption, it is desirable to operate the amplifier as a class AB amplifier, in which the current in the output devices is small when the output is near a crossover voltage. The crossover voltage may be an analog ground reference voltage about halfway between positive and negative power supply voltages. Crossover distortion occurs when the time-varying output signal is near the crossover voltage, because the gain characteristic of the output stage providing the output signal is non-linear around the crossover voltage, and the output signal shows a similar distortion.
For example, a common-source class AB output stage comprises two large CMOS transistors coupled in series between the positive and negative power supply voltage terminals. In class AB operation, the transistors in the output stage usually have at least some minimum bias applied to them, so that the gate-to-source voltage (V.sub.GS) of the transistors will be slightly greater than their threshold voltages, and any nonlinearities in the current/voltage characteristics are minimized. Also, class AB operation provides a small DC quiescent current and thus reduces power consumption. If the V.sub.GS of each of the transistors is slightly greater than its respective threshold voltage, a small standby current flows in the output stage when the output signal is at crossover. One way the transistors may be kept partially conductive is by clamping the voltage on their gates at a V.sub.GS of slightly greater than a threshold voltage.
Clamping circuits may be used to thus maintain low power and low crossover distortion, and known clamping circuits function quite well for larger geometry integrated circuit technology. Transistors inside the clamping circuit provide clamp voltages for the transistors in the output stage. Since the transistors in the output stage are large, it is desirable for the transistors in the clamping circuit to be smaller to avoid high power consumption and increased circuit area. The clamp voltages are maintained by matching the current densities between the transistors in the output stage and the smaller-sized transistors in the clamping circuit. However, as channel lengths become smaller, approaching one micron and below, channel length modulation becomes more significant. Channel length modulation may cause the density of the current flowing in the output stage transistor to vary by as much as an order of magnitude from the density of the current flowing in the clamping circuit transistors. Thus, with short channel length transistors, known clamping circuits no longer provide an accurate clamp voltage for the transistors in the output stage, and crossover distortion and high currents again become a problem.